1. Field of the Invention
The present invention relates to an elastic wave filter device including an elastic wave filter and a parallel trap that are connected to each other in a ladder pattern on a piezoelectric substrate, and also relates to a module including the elastic wave filter device.
2. Description of the Related Art
Elastic wave filter devices that use electromechanical vibration, such as surface elastic waves using vibration on a surface of a piezoelectric substrate, bulk acoustic waves using vibration inside a piezoelectric substrate, and boundary acoustic waves using vibration on an interface of a laminated piezoelectric substrate, are used for RF filters or the like of mobile phone apparatuses.
An example of the elastic wave filter device includes a device in which a plurality of one-port elastic wave resonators (hereinafter simply referred to as resonators) are arranged in an elastic wave propagation direction and are longitudinally coupled to form a multi-mode filter, and trap resonators are inserted into the multi-mode filter in series and in parallel to form a ladder-pattern connection (e.g., see Japanese Unexamined Patent Application Publication No. 7-131290).
FIG. 1 is a circuit diagram illustrating an example configuration of a conventional elastic wave filter device.
This elastic wave filter device includes a multi-mode filter 111, which is made up of a plurality of resonators 105 and a plurality of resonators 106, and also includes series trap resonators 101, 102, 107, and 108 and parallel trap resonators 103, 104, 109, and 110.
In the plurality of resonators 105, individual first IDT electrodes are connected to input lines, and individual second IDT electrodes are connected to ground lines. In the plurality of resonators 106, individual first IDT electrodes are connected to output lines, and individual second IDT electrodes are connected to ground lines. The plurality of resonators 105 and 106 are alternately arranged in an elastic wave propagation direction and are longitudinally coupled, thereby forming the multi-mode filter 111.
The series trap resonators 101 and 102 are inserted in series into an input line, and the series trap resonators 107 and 108 are inserted in series into an output line. The parallel trap resonators 103 and 104 are inserted in parallel between input lines and ground lines, and the parallel trap resonators 109 and 110 are inserted in parallel between output lines and ground lines.
Such an elastic wave filter device is normally formed as a chip that is integrally provided on a piezoelectric substrate. The chip of this elastic wave filter device is mounted on a module body, such as a ceramic substrate or a printed board, so as to be formed as a module. At that time, it is difficult to separately provide the ground connection electrodes represented by the plurality of ground symbols illustrated in FIG. 1 on the surface of the piezoelectric substrate in view of the limited chip size. Thus, as illustrated in the schematic view in FIG. 2, all the ground lines are commonly connected to a single ground connection electrode.
In a case where an elastic wave filter device has a module structure, the wiring on a module body extending from external terminals of the module to the elastic wave filter device has a parasitic impedance.
Therefore, a ground connection electrode provided on a surface of a piezoelectric substrate is connected to an external ground terminal of the module via the parasitic impedance. However, the effect of the parasitic impedance may degrade the attenuation characteristic and isolation characteristic of an elastic wave filter.